Realizing final control elements by piezoelectric elements in practice proves to be advantageous particularly if the respective final control element must perform quick and/or frequent movements.
Using piezoelectric elements as a final control element proves to be advantageous in fuel injection nozzles for internal combustion engines, among other uses. The usability of piezoelectric elements in fuel injection nozzles, is described in, for example, to European Patents EP 0 371 469 B1 and EP 0 379 182 B1.
Piezoelectric elements are capacitive consumers, which, as described above, contract or expand, depending on the charge state in each case, i.e., depending on the emerging or applied voltage.
The charging and discharging of a piezoelectric element can occur, among other ways, via a component having inductive properties, such as a coil, this coil primarily serving to limit the charging current occurring during charging as well as the discharging current occurring during discharging. Such an arrangement is shown in FIG. 8.
The piezoelectric element to be charged or to be discharged is referred to in FIG. 8 with the reference symbol 101. It is 10 part of a charging circuit, which can be closed by a charging switch 102, and of a discharging circuit, which can be closed by a discharging switch 106. The charging circuit includes a charging switch 102, a diode 103, a charging coil 104, piezoelectric element 101 and a voltage source 105, connected in series. The discharging circuit includes a discharging switch 106, a diode 107, a discharging coil 108 and piezoelectric element 101, connected in series.
Diode 103 of the charging circuit prevents the flow of a current in the charging circuit, which would discharge the piezoelectric element. Diode 103 and charging switch 102 can be executed together as a semiconductor switch.
Diode 107 of the discharging circuit prevents the flow of a current in the discharging circuit, which would charge the piezoelectric element. Diode 107 and charging switch 106 can be executed together as a semiconductor switch.
When the normally opened charging switch 102 is closed, then a charging current flows in the charging circuit, by which piezoelectric element 101 is charged. The charge stored in piezoelectric element 101, i.e., as a result, the voltage emerging at it, and consequently, the current exterior dimensions of piezoelectric element 101, basically remain unchanged upon charging it (the piezoelectric element).
When the normally opened discharging switch 106 is closed, then a discharging current flows in the discharging circuit, by which piezoelectric element 101 is discharged. The charge state of piezoelectric element 101, i.e., as a result, the voltage emerging at it, and consequently, the current exterior dimensions of piezoelectric element 101, basically remain unchanged upon discharging it (the piezoelectric element).
Such a charging and discharging of piezoelectric elements is advantageous since it can occur with only a small dissipation of power and only a relatively low development of heat for lack of ohmic resistances worth mentioning in the charging circuit and the discharging circuit.
On the other hand, the scale and the time characteristic of the charging and discharging are not always ideal. Disturbances primarily include charging and discharging rates varying in time, more or less distinct transient reactions as well as an only partial or excessive charging and/or discharging of the piezoelectric element, by which even a charging with opposite polarity can occur during discharging.
It should be clear that this not only represents a considerable stress for the piezoelectric element to be charged or discharged, but can also impair its usage to the intended purpose.